Many spectral sensitizing dyes are known in the field of electrophotographic photosensitive layers for a photoconductive material-resin dispersion system. These spectral sensitizing dyes must have various properties and among them, it is particularly important for the dyes to be adsorbed well on the photoconductive materials, for the dyes to have high sensitizing efficiency, and for the dyes to not excessively reduce the dark resistance of electrophotographic photosensitive materials. Examples of the dyes satisfying these conditions are described in U.S. Pat. Nos. 3,052,540, 3,110,591, 3,125,447, 3,128,179, 3,132,942, 3,241,959 and 3,121,008 and British Pat. No. 1,093,823.
Spectral sensitizing dyes for red light to infrared rays are described in U.S. Pat. Nos. 3,619,154 and 3,682,630 but these dyes have severe practical faults because they tend to decompose and hence they are greatly decomposed during storage of the dyes or during production or storage of the electrophotographic photosensitive layers containing the dyes, thereby the properties of the photosensitive layers are deteriorated. Harazaki et at. in Kogyo Kagaku Zasshi, Vol. 66, No. 2, 26 (1963) state that sensitizing dyes for red light to infrared rays are unstable in comparison with sensitizing dyes for light (visible light) having shorter wavelengths than above.